Porous metal-organic frameworks are known in the prior art and form an interesting class of substances which can be an alternative to organic zeolites for various applications.
Numerous processes have been developed for preparing such porous metal-organic frameworks. Typically, a metal salt is reacted with the at least bidentate organic compound, for example a dicarboxylic acid, in a suitable solvent under superatmospheric pressure and elevated temperature.
However, difficulties frequently occur here. One problem can be that, owing to the use of a metal salt, the counterion to the metal cation remaining in the reaction medium after formation of the metal-organic framework (for example nitrate) has to be separated from the framework.
The use of high pressures and temperatures places severe demands on the synthesis apparatus for preparing a porous metal-organic framework. Usually, only a batch synthesis in comparatively small apparatuses is possible and has been described. A scale-up is found to be very complicated.
A further difficulty is that, depending on the metal and organic compound used for preparing the framework, it is not possible to carry over the reaction conditions readily. Such a case occurs, for example, when the metal component of the metal-organic framework is a main group metal of the second or third main group of the Periodic Table. Here, significantly different reaction conditions compared to analogous frameworks in which the metal component is a transition metal, for example zinc or copper, are sometimes employed for the preparation.
Such porous metal-organic frameworks, which can have a main group metal of the second or third main group, also differ in respect of their properties from the abovementioned analogous frameworks, which could be a reason why modified preparative processes are frequently employed for this purpose in the prior art.
WO-A 2007/023134 describes the preparation of such metal-organic frameworks based on main group metals. Here, preparation in a nonaqueous medium is disclosed. Although the synthesis proposed brings advantages, the use of organic solvents as reaction medium remains problematical, in particular for reactions of relatively large quantities of starting materials.
WO-A 2007/118841 likewise describes the preparation of a framework based on aluminum fumarate in organic solvents.
Apart from the problems associated with the use of organic solvents for health and environmental reasons, the processes disclosed in the prior art have conditions which tend to be unsuitable for production on an industrial scale and also in respect of characteristic parameters such as the space-time yield.
There is therefore a need for improved processes which, in particular, are suitable for industrial or large-scale production.